Return a set instead of a vector for the miner fund whitelist

Summary:
This will make writing a robust implementation of GetMinerFundWhitelist easier
as we add another path for a policy whitelist.

Also took this opportunity to use auto where appropriate.

Test Plan:

ninja check-functional

Reviewers: #bitcoin_abc, PiRK, Fabien

Reviewed By: #bitcoin_abc, PiRK, Fabien

Subscribers: Fabien, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13094